Call old man up in port (5)
I believe the answer is:
radio
'call' is the definition.
'old man up in port' is the wordplay.
'old man' becomes 'da' (da can mean dad).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates putting letters inside.
'port' becomes 'rio' (I've seen this before).
'da' reversed gives 'ad'.
'ad' put inside 'rio' is 'RADIO'.
(Other definitions for radio that I've seen before include "Transistor" , "send a message" , "Wireless broadcast" , "Electronic receiver" , "Communications receiver" .)